(a) Field of the Invention
The present invention relates to an unleaded surface-treated steel sheet for fuel tanks of an automobile, more specifically, a resin-coated steel sheet coated with resin film on the surface of unleaded surface-treated steel sheet coated with chromate layer and a method of fabricating the same
(b) Description of the Related Art
Generally, it is required that fuel tanks for storing fuel of an automobile should have resistance to corrosion on its outer surface to be exposed to the atmosphere (hereinafter, referred to as the xe2x80x9ccosmetic corrosion resistancexe2x80x9d) as well as resistance to corrosion on its inner surface to contact fuel such as gasoline (hereinafter, referred to as the xe2x80x9cfuel corrosion resistancexe2x80x9d).
In addition, since two parts are joined each other in a fuel tank by seam welding or joining method such as brazing, good weldability and good processibility are required for the steel sheet to be applied for use in forming fuel tanks.
A ternesheet that is a cold-rolled steel sheet plated with lead-tin (Pbxe2x80x94Sn) alloy has widely used as such a steel sheet for fuel tanks. However, the ternesheet has been limited in its use because it contains lead (Pb) that causes environmental pollution.
In this connection, extensive researches have been made to develop a surface-treated steel sheet for fuel tanks without any lead content.
Japanese patent laid-open No.63-69631 and Japanese patent publication No.2-18982 described an electric zinc plated steel sheet as an unleaded steel sheet. The unleaded steel sheets described in the above patent were prepared by coating zinc or zinc-based alloy such as Znxe2x80x94Ni, Znxe2x80x94Co, Znxe2x80x94Fe, Znxe2x80x94Al on the surface of steel sheet in the amount of 1-200 g/m2 and coating organic resin film comprising phenoxy resin and rubber-modified epoxy on the upper side of zinc plated layer in the thickness of 2-50 xcexcm.
The above surface-treated steel sheet, however, has a problem such as exfoliation of plated layer while processing because the plated layer is thick. In addition, since an organic coating layer coated on the uppermost layer is also thick, it is difficult to weld. Furthermore, adhesion between zinc alloy plated layer and organic resin coated layer becomes lower so that there is a problem of exfoliation between the two layers.
In order to solve the above problems, the present inventors invented an International patent publication No. WO 00/32843. The above patent provides an unleaded plated steel sheet coated with resin coating comprising metal powder on chromate layer that is in turn coated on zinc or zinc-based alloy plated steel sheet.
The above invention, however, has a problem that since processed side does not resist against the pressure of mold die while processing due to metal powder comprised in resin coating, it may exfoliate a part of resin layer
In the coated resin coating, the above problem results from lower slip characteristic of resin layer. The slip characteristic of resin layer has a close relation with wax added to resin solution, and a type, composition and particle size of metallic powder.
Therefore, an unleaded steel sheet that can prevent exfoliation of coating by improving slip characteristic as well as maintaining cosmetic corrosion resistance and fuel corrosion resistance is required.
Therefore, the present invention is derived to solve the above problems. The present invention provides an improved resin solution used for preparing resin-coated steel sheet without damaging chemical characteristics of resin.
It is another object of the present invention to provide a method of fabricating resin-coated steel sheet for fuel tanks of an automobile which has improved cosmetic corrosion resistance and fuel corrosion resistance as well as processibility by coating the resin solution.
In order to achieve the above objects, the resin solution of the present invention is prepared by mixing a main solution selected from epoxy resin, urethane resin, and phenoxy resin melaniine resin, colloidal silica, polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d)-based wax, and at least one metallic powder selected from Al, Zn, Mn, Co, Ni, Sn, and SnO.;
The main solution used in the resin solution of the present invention is a water-soluble phenoxy resin with a number average molecular weight of 25,000-50,000. Melamine resin is added as a hardener and the amount is 2-15 phr on the basis of the main solution. Also, an amount of colloidal silica is 10-20 phr on the basis of the main solution, an amount of PTFE wax is 2-10 phr on the basis of the main solution, and an amount of metallic powder is 5-70 phr on the basis of the main solution. 
The PTFE-based wax added to the resin solution of the present invention is preferably a powder type and particle size of 0.1-3 xcexcm. In addition, particle size of the metallic powder is 0.5-5 xcexcm.
The present invention provides a method of fabricating surface-treated steel sheet by using the resin solution.
The surface-treated steel sheet of the present invention comprises coating chromate film on a cold-rolled steel sheet plated with zinc or zinc alloy and then it is coated with resin solution of the present invention. After coating the resin solution, it is baking dried at 140-250xc2x0 C. based on metal temperature. In this case, the coating thickness of resin solution is preferably 1-10 xcexcm based on coating thickness after drying.
The surface-treated steel sheet of the present invention can maintain slip characteristic of the resin coating as well as prevent exfoliation of coating resulted from metallic powder.
Therefore, if the surface-treated steel sheet of the present invention is used to manufacture a fuel tank, a press processibility of the steel sheet is remarkably improved.